SYNTHESIS, CONFORMATIONAL PROPERTIES, AND IMMUNOGENICITY OF A CYCLIC TEMPLATE-BOUND PEPTIDE MIMETIC CONTAINING AN NPNA MOTIF FROM THE CIRCUMSPOROZOITE PROTEIN OF PLASMODIUM-FALCIPARUM

The immunodominant central portion of the circumsporozoite (CS) surfac e protein of the malaria parasite Plasmodium falciparum contains a tet rapeptide motif, Asn-Pro-Asn-Ala (NPNA), tandemly repeated almost 40 t imes. The three-dimensional structure of the CS protein, including the central repeat region, is presently unknown. We have investigated an approach to stabilize beta-turns in a single NPNA motif, by its incorp oration into a template-bound cyclic peptide comprising the sequence A NPNAA. The template was designed to stabilize beta-turns in the peptid e loop and to allow its conjugation to T-cell epitopes in a multiple a ntigen-peptide. NMR studies and MD simulations with time-averaged NOE- derived upper distance restraints support the formation of a stable be ta-I turn conformation in the NPNA motif of this template-bound antige n. Balb/c mice immunized with a multiple-antigen-peptide containing fo ur copies of the template-bound loop conjugated to a single universal T-cell epitope produced antibodies that bound P. falciparum sporozoite s in immunofluorescence assays. These results provide further support for the immunological relevance of a type-I beta-turn conformation bas ed on the NPNA cadence in the repeat region of the CS protein and illu strate the use of a novel template for the evaluation of conformationa lly constrained peptide immunogens.